Al fondo del salón
'Classroom' is 'aula' in Spanish.
The Spanish word for "classroom" is "aula." It is pronounced, "AH-oo-lah." Sites such as learn-spanish.co.il provide audio pronunciations of many common Spanish words.
You could say "¿Cuántas sillas hay en la clase?"
tengo una pizarra en mi salón de clases
I sleep on my back in Spanish is: Yo duermo de espaldas.
aula
Me retracto
Devuelvaselo
we say: necesito que vuelvas
If you mean your back as in the part of your anatomy, the spanish word is "espalda" (S-paul-dah)
classroom
no a mis espaldas